Compression ignition engines, such as diesel engines, provide advantages in fuel economy, but produce and emit both NOx (nitrogen oxides) and particulates during normal operation. When primary measures (actions that affect the combustion process itself, e.g., exhaust gas recirculation and engine timing adjustments) are taken to reduce one, the other is usually increased. Thus, combustion conditions selected to reduce pollution from particulates and obtain good fuel economy tend to increase the output of NOx. Current and proposed regulations and legislation challenge manufacturers to achieve good fuel economy while at the same time require the reduction of the emissions of particulates and NOx.
In order to meet such requirements or restrictions a method known as SCR (selective catalytic reduction) has been used for reducing the emission of NOx. The SCR method consists of injecting gaseous ammonia NH3, ammonia in aqueous solution or aqueous urea, or ammonia supplied from an ammonia generator using a solid source of ammonia such as ammonia carbamate or ammonia carbanate, into the exhaust gas system of the compression ignition engine as a reduction agent. When the temperature of the exhaust gas stream is above a reaction temperature, for example a temperature above 160° C. for aqueous urea, the reduction agent undergoes a hydrolysis process and is decomposed into ammonia and CO2. As the exhaust gas stream is passed through the SCR catalyst the gaseous ammonia reacts with the NOx to reduce the NOx to molecular nitrogen. This reduces or limits the NOx emissions from the compression ignition engine.
Although an SCR catalyst system is effective in reducing NOx emissions above a certain exhaust gas temperature, it has been found that the effectiveness of the SCR catalyst is drastically reduced below this temperature due to several factors. One factor is that the rate of NOx conversion is strongly affected by the temperature of the exhaust gas. The NOx conversion efficiency drops off quickly when the exhaust gas temperature is below the temperature at which the conversion efficiency is 50%. This is known as the catalyst light-off temperature. Consequently, the SCR catalyst is not effective when the engine is operating under light load conditions.
When urea solution is used, another factor which reduces the effectiveness of the SCR catalyst system is the minimum hydrolysis temperature of the urea solution. Whenever a solution of urea is used to supply ammonia to the SCR catalyst, it is important that the temperature of the exhaust gas stream be at or above the minimum hydrolysis temperature, which is 160° C. Below its minimum hydrolysis temperature, the urea solution does not decompose into ammonia at a fast enough rage for a typical engine application. If the unhydrolyzed solution of urea is injected into the SCR catalyst, some of the urea is deposited on the surface of the SCR catalyst as a solid residue, which results in the system clogging or plugging. These conditions occur when an engine is operated under transient conditions, or under light load conditions, and NOx reduction by the SCR catalyst can not be achieved.
A third factor that reduces the efficiency of an SCR catalyst system is that the surface temperature of the catalyst must be at a minimum temperature. Until this temperature is obtained, the solution of urea should not be injected into the catalyst, and the delay in injecting the solution of urea causes the SCR catalyst system to lose efficiency. Low catalyst surface temperature can exist when the engine is coming out of a cold start condition or out of a prolonged period of light load operation.
One other factor that reduces the efficiency of the SCR catalyst system occurs when the engine is operating under a light load or an idle condition, and combustion takes place at low in-cylinder temperatures. Some of the unburned fuel and lube oil can survive the combustion process in the engine and be discharged into the exhaust gas stream as liquid droplets. Since the exhaust gas temperature is low, some of the liquid droplets are deposited on the surface of the SCR catalyst. A subsequent increase in exhaust temperature causes the liquid hydrocarbons to undergo partial oxidation. The residue formed can block the micro pores of the catalyst washcoat, causing the catalyst to reduce efficiency through loss of surface area within the catalyst. Increasing the combustion temperature is an effective way to reduce the amount of liquid hydrocarbons entering into the SCR catalyst.
In view of these factors which decrease the efficiency of the SCR catalyst, it would be advantageous to have a system that could insure that the temperature of the exhaust gas is at or above the minimum hydrolysis temperature for the solution of urea. It would also be desirable to have an emissions control system that ensures that the SCR catalyst does not become clogged with liquid hydrocarbons, or other matter that reduces the capability of the SCR catalyst to operate correctly or properly.